In recent years, much work has been done to design spacers for insulating glass units. Metal spacers between opposing panes can conduct heat efficiently so that the insulating performance is substantially reduced. Several alternative spacer designs have been described in Popular Science, August 1992 at page 46. Another alternative spacer design which has been developed to provide a highly desirable combination of structural integrity, moisture and gas impermeability and thermal efficiency is described in EP 475,213. This spacer utilizes a metal U-channel separated from the glass panels and the exterior edge of the assembly by a thermal brake. The U-channel opens to the interior of the assembly and includes a moisture permeable adhesive having a desiccant therein on the inner bottom surface of the channel. Adhesive materials having a permeability of greater than 2 gm mm/M.sup.2 day as determined by ASTM F-372-73 are recommended. The desiccant loaded adhesive material contemplated for use in the invention has been a polyurethane-based formulation which is formed into a soft string and applied to the U-channel.
It would be an advantage to have a pumpable desiccant loaded formulation which could be more easily applied to the U-channel of the EP 475,213 reference and which would effectively desiccate the insulating glass assembly over its useful life.
Insulating glass assemblies can be subject to accumulation of unsightly chemical "fog" on the interior surface of the glass panels. Fogging can be caused by off-gassing of organic materials in the spacer or from other structures in the interior of the insulating glass assembly such as painted decorative grids. Off-gassing can be caused, for instance, by incompletely dry coatings on such grids or by UV or thermally induced degradation of a polymer during the service life of the insulating glass assembly. Any desiccant loaded material which is utilized in the U-channel of EP 475,213 should not contribute to chemical fogging and, desirably, should effectively adsorb organics originating from other structures within the insulating glass assembly so as to reduce chemical fogging.
The desiccant containing adhesive material utilized in the U-channel structure described in EP 475,213 must also be able to withstand elevated temperature without sagging or creeping and must maintain adhesion to the channel during the service life of the unit. Because the U-channel is open, any sagging, creeping or peeling of the desiccant composition will be readily visible. Cracking or crazing of the desiccant composition due to UV light degradation would also be readily visible.
These numerous requirements severely limit the options available for formulating a pumpable desiccant loaded material suitable for use in spacer structures of the type disclosed in EP 475,213.
In U.S. Pat. No. 5,510,416, the inventors of the present application disclose thermoplastic hot-melt mastic compositions which simultaneously meet rigorous pumpability, non-fogging, desiccant loading, and non-sag requirements of the open U-channel spacer structure of EP 475,213. Such compositions suitably include a resin component consisting essentially of 4-30 parts by weight of the resin component of a film former selected from the group consisting of polyisobutylene polymers having a weight average molecular weight in the range of about 35,000 to about 60,000, and mixtures of such polymers with minor amounts of a butyl rubber having a weight average molecular weight of up to about 500,000; 20-50 parts by weight of the resin component of a non-crystalline homopolymer, copolymer, terpolymer or graft copolymer comprising polypropylene; and, 0-20 parts by weight of the resin component of a low volatile UV stable tackifier compatible with the film former and polypropylene components; and an adsorbent component which includes at least 20% by weight of the composition of a moisture or moisture and volatile organic chemical adsorbing material; and 0-10% by weight of the composition of an adsorbent of volatile organic compounds.